[TCP] BIC: CUBIC window growth (2.0)

Replace existing BIC version 1.1 with new version 2.0.
The main change is to replace the window growth function
with a cubic function as described in:
  http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdfSigned-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/Kconfig

@@ -456,6 +456,14 @@ config TCP_CONG_BIC
 	increase provides TCP friendliness.
 	See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/
 
+config TCP_CONG_CUBIC
+	tristate "CUBIC TCP"
+	default m
+	---help---
+	This is version 2.0 of BIC-TCP which uses a cubic growth function
+	among other techniques.
+	See http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf
+
 config TCP_CONG_WESTWOOD
 	tristate "TCP Westwood+"
 	default m

net/ipv4/Makefile

@@ -34,6 +34,7 @@ obj-$(CONFIG_INET_DIAG) += inet_diag.o
 obj-$(CONFIG_IP_ROUTE_MULTIPATH_CACHED) += multipath.o
 obj-$(CONFIG_INET_TCP_DIAG) += tcp_diag.o
 obj-$(CONFIG_TCP_CONG_BIC) += tcp_bic.o
+obj-$(CONFIG_TCP_CONG_CUBIC) += tcp_cubic.o
 obj-$(CONFIG_TCP_CONG_WESTWOOD) += tcp_westwood.o
 obj-$(CONFIG_TCP_CONG_HSTCP) += tcp_highspeed.o
 obj-$(CONFIG_TCP_CONG_HYBLA) += tcp_hybla.o

net/ipv4/tcp_cubic.c

+/*
+ * TCP CUBIC: Binary Increase Congestion control for TCP v2.0
+ *
+ * This is from the implementation of CUBIC TCP in
+ * Injong Rhee, Lisong Xu.
+ *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant
+ *  in PFLDnet 2005
+ * Available from:
+ *  http://www.csc.ncsu.edu/faculty/rhee/export/bitcp/cubic-paper.pdf
+ *
+ * Unless CUBIC is enabled and congestion window is large
+ * this behaves the same as the original Reno.
+ */
+
+#include <linux/config.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <net/tcp.h>
+
+
+#define BICTCP_BETA_SCALE    1024	/* Scale factor beta calculation
+					 * max_cwnd = snd_cwnd * beta
+					 */
+#define BICTCP_B		4	 /*
+					  * In binary search,
+					  * go to point (max+min)/N
+					  */
+#define	BICTCP_HZ		10	/* BIC HZ 2^10 = 1024 */
+
+static int fast_convergence = 1;
+static int max_increment = 16;
+static int beta = 819;		/* = 819/1024 (BICTCP_BETA_SCALE) */
+static int initial_ssthresh = 100;
+static int bic_scale = 41;
+static int tcp_friendliness = 1;
+
+module_param(fast_convergence, int, 0644);
+MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
+module_param(max_increment, int, 0644);
+MODULE_PARM_DESC(max_increment, "Limit on increment allowed during binary search");
+module_param(beta, int, 0644);
+MODULE_PARM_DESC(beta, "beta for multiplicative increase");
+module_param(initial_ssthresh, int, 0644);
+MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
+module_param(bic_scale, int, 0644);
+MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
+module_param(tcp_friendliness, int, 0644);
+MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
+
+
+/* BIC TCP Parameters */
+struct bictcp {
+	u32	cnt;		/* increase cwnd by 1 after ACKs */
+	u32 	last_max_cwnd;	/* last maximum snd_cwnd */
+	u32	loss_cwnd;	/* congestion window at last loss */
+	u32	last_cwnd;	/* the last snd_cwnd */
+	u32	last_time;	/* time when updated last_cwnd */
+	u32	bic_origin_point;/* origin point of bic function */
+	u32	bic_K;		/* time to origin point from the beginning of the current epoch */
+	u32	delay_min;	/* min delay */
+	u32	epoch_start;	/* beginning of an epoch */
+	u32	ack_cnt;	/* number of acks */
+	u32	tcp_cwnd;	/* estimated tcp cwnd */
+#define ACK_RATIO_SHIFT	4
+	u32	delayed_ack;	/* estimate the ratio of Packets/ACKs << 4 */
+};
+
+static inline void bictcp_reset(struct bictcp *ca)
+{
+	ca->cnt = 0;
+	ca->last_max_cwnd = 0;
+	ca->loss_cwnd = 0;
+	ca->last_cwnd = 0;
+	ca->last_time = 0;
+	ca->bic_origin_point = 0;
+	ca->bic_K = 0;
+	ca->delay_min = 0;
+	ca->epoch_start = 0;
+	ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
+	ca->ack_cnt = 0;
+	ca->tcp_cwnd = 0;
+}
+
+static void bictcp_init(struct sock *sk)
+{
+	bictcp_reset(inet_csk_ca(sk));
+	if (initial_ssthresh)
+		tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
+}
+
+/* 65536 times the cubic root */
+static const u64 cubic_table[8]
+	= {0, 65536, 82570, 94519, 104030, 112063, 119087, 125367};
+
+/*
+ * calculate the cubic root of x
+ * the basic idea is that x can be expressed as i*8^j
+ * so cubic_root(x) = cubic_root(i)*2^j
+ *  in the following code, x is i, and y is 2^j
+ *  because of integer calculation, there are errors in calculation
+ *  so finally use binary search to find out the exact solution
+ */
+static u32 cubic_root(u64 x)
+{
+        u64 y, app, target, start, end, mid, start_diff, end_diff;
+
+        if (x == 0)
+                return 0;
+
+        target = x;
+
+        /* first estimate lower and upper bound */
+        y = 1;
+        while (x >= 8){
+                x = (x >> 3);
+                y = (y << 1);
+        }
+        start = (y*cubic_table[x])>>16;
+        if (x==7)
+                end = (y<<1);
+        else
+                end = (y*cubic_table[x+1]+65535)>>16;
+
+        /* binary search for more accurate one */
+        while (start < end-1) {
+                mid = (start+end) >> 1;
+                app = mid*mid*mid;
+                if (app < target)
+                        start = mid;
+                else if (app > target)
+                        end = mid;
+                else
+                        return mid;
+        }
+
+        /* find the most accurate one from start and end */
+        app = start*start*start;
+        if (app < target)
+                start_diff = target - app;
+        else
+                start_diff = app - target;
+        app = end*end*end;
+        if (app < target)
+                end_diff = target - app;
+        else
+                end_diff = app - target;
+
+        if (start_diff < end_diff)
+                return (u32)start;
+        else
+                return (u32)end;
+}
+
+static inline u32 bictcp_K(u32 dist, u32 srtt)
+{
+        u64 d64;
+        u32 d32;
+        u32 count;
+        u32 result;
+
+        /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
+           so K = cubic_root( (wmax-cwnd)*rtt/c )
+           the unit of K is bictcp_HZ=2^10, not HZ
+
+           c = bic_scale >> 10
+           rtt = (tp->srtt >> 3 ) / HZ
+
+           the following code has been designed and tested for
+           cwnd < 1 million packets
+           RTT < 100 seconds
+           HZ < 1,000,00  (corresponding to 10 nano-second)
+
+        */
+
+        /* 1/c * 2^2*bictcp_HZ */
+        d32 = (1 << (10+2*BICTCP_HZ)) / bic_scale;
+        d64 = (__u64)d32;
+
+        /* srtt * 2^count / HZ
+           1) to get a better accuracy of the following d32,
+           the larger the "count", the better the accuracy
+           2) and avoid overflow of the following d64
+           the larger the "count", the high possibility of overflow
+           3) so find a "count" between bictcp_hz-3 and bictcp_hz
+           "count" may be less than bictcp_HZ,
+           then d64 becomes 0. that is OK
+        */
+        d32 = srtt;
+        count = 0;
+        while (((d32 & 0x80000000)==0) && (count < BICTCP_HZ)){
+                d32 = d32 << 1;
+                count++;
+        }
+        d32 = d32 / HZ;
+
+        /* (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)  */
+        d64 = (d64 * dist * d32) >> (count+3-BICTCP_HZ);
+
+        /* cubic root */
+        d64 = cubic_root(d64);
+
+        result = (u32)d64;
+        return result;
+}
+
+/*
+ * Compute congestion window to use.
+ */
+static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
+{
+	u64 d64;
+	u32 d32, t, srtt, bic_target, min_cnt, max_cnt;
+
+	ca->ack_cnt++;	/* count the number of ACKs */
+
+	if (ca->last_cwnd == cwnd &&
+	    (s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
+		return;
+
+	ca->last_cwnd = cwnd;
+	ca->last_time = tcp_time_stamp;
+
+	srtt = (HZ << 3)/10;	/* use real time-based growth function */
+
+	if (ca->epoch_start == 0) {
+		ca->epoch_start = tcp_time_stamp;	/* record the beginning of an epoch */
+		ca->ack_cnt = 1;			/* start counting */
+		ca->tcp_cwnd = cwnd;			/* syn with cubic */
+
+		if (ca->last_max_cwnd <= cwnd) {
+			ca->bic_K = 0;
+			ca->bic_origin_point = cwnd;
+		} else {
+			ca->bic_K = bictcp_K(ca->last_max_cwnd-cwnd, srtt);
+			ca->bic_origin_point = ca->last_max_cwnd;
+		}
+	}
+
+        /* cubic function - calc*/
+        /* calculate c * time^3 / rtt,
+         *  while considering overflow in calculation of time^3
+	 * (so time^3 is done by using d64)
+	 * and without the support of division of 64bit numbers
+	 * (so all divisions are done by using d32)
+         *  also NOTE the unit of those veriables
+         *	  time  = (t - K) / 2^bictcp_HZ
+         *	  c = bic_scale >> 10
+	 * rtt  = (srtt >> 3) / HZ
+	 * !!! The following code does not have overflow problems,
+	 * if the cwnd < 1 million packets !!!
+         */
+
+	/* change the unit from HZ to bictcp_HZ */
+        t = ((tcp_time_stamp + ca->delay_min - ca->epoch_start)
+	     << BICTCP_HZ) / HZ;
+
+        if (t < ca->bic_K)		/* t - K */
+                d32 = ca->bic_K - t;
+        else
+                d32 = t - ca->bic_K;
+
+        d64 = (u64)d32;
+        d32 = (bic_scale << 3) * HZ / srtt;			/* 1024*c/rtt */
+        d64 = (d32 * d64 * d64 * d64) >> (10+3*BICTCP_HZ);	/* c/rtt * (t-K)^3 */
+        d32 = (u32)d64;
+        if (t < ca->bic_K)                                	/* below origin*/
+                bic_target = ca->bic_origin_point - d32;
+        else                                                	/* above origin*/
+                bic_target = ca->bic_origin_point + d32;
+
+        /* cubic function - calc bictcp_cnt*/
+        if (bic_target > cwnd) {
+		ca->cnt = cwnd / (bic_target - cwnd);
+        } else {
+                ca->cnt = 100 * cwnd;              /* very small increment*/
+        }
+
+	if (ca->delay_min > 0) {
+		/* max increment = Smax * rtt / 0.1  */
+		min_cnt = (cwnd * HZ * 8)/(10 * max_increment * ca->delay_min);
+		if (ca->cnt < min_cnt)
+			ca->cnt = min_cnt;
+	}
+
+        /* slow start and low utilization  */
+	if (ca->loss_cwnd == 0)		/* could be aggressive in slow start */
+		ca->cnt = 50;
+
+	/* TCP Friendly */
+	if (tcp_friendliness) {
+		u32 scale = 8*(BICTCP_BETA_SCALE+beta)/3/(BICTCP_BETA_SCALE-beta);
+		d32 = (cwnd * scale) >> 3;
+	        while (ca->ack_cnt > d32) {		/* update tcp cwnd */
+	                ca->ack_cnt -= d32;
+        	        ca->tcp_cwnd++;
+		}
+
+		if (ca->tcp_cwnd > cwnd){	/* if bic is slower than tcp */
+			d32 = ca->tcp_cwnd - cwnd;
+			max_cnt = cwnd / d32;
+			if (ca->cnt > max_cnt)
+				ca->cnt = max_cnt;
+		}
+        }
+
+	ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
+	if (ca->cnt == 0)			/* cannot be zero */
+		ca->cnt = 1;
+}
+
+
+/* Keep track of minimum rtt */
+static inline void measure_delay(struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	struct bictcp *ca = inet_csk_ca(sk);
+	u32 delay;
+
+	/* No time stamp */
+	if (!(tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr) ||
+	     /* Discard delay samples right after fast recovery */
+	    (s32)(tcp_time_stamp - ca->epoch_start) < HZ)
+		return;
+
+	delay = tcp_time_stamp - tp->rx_opt.rcv_tsecr;
+	if (delay == 0)
+		delay = 1;
+
+	/* first time call or link delay decreases */
+	if (ca->delay_min == 0 || ca->delay_min > delay)
+		ca->delay_min = delay;
+}
+
+static void bictcp_cong_avoid(struct sock *sk, u32 ack,
+			      u32 seq_rtt, u32 in_flight, int data_acked)
+{
+	struct tcp_sock *tp = tcp_sk(sk);
+	struct bictcp *ca = inet_csk_ca(sk);
+
+	if (data_acked)
+		measure_delay(sk);
+
+	if (!tcp_is_cwnd_limited(sk, in_flight))
+		return;
+
+	if (tp->snd_cwnd <= tp->snd_ssthresh)
+		tcp_slow_start(tp);
+	else {
+		bictcp_update(ca, tp->snd_cwnd);
+
+		/* In dangerous area, increase slowly.
+		 * In theory this is tp->snd_cwnd += 1 / tp->snd_cwnd
+		 */
+		if (tp->snd_cwnd_cnt >= ca->cnt) {
+			if (tp->snd_cwnd < tp->snd_cwnd_clamp)
+				tp->snd_cwnd++;
+			tp->snd_cwnd_cnt = 0;
+		} else
+			tp->snd_cwnd_cnt++;
+	}
+
+}
+
+static u32 bictcp_recalc_ssthresh(struct sock *sk)
+{
+	const struct tcp_sock *tp = tcp_sk(sk);
+	struct bictcp *ca = inet_csk_ca(sk);
+
+	ca->epoch_start = 0;	/* end of epoch */
+
+	/* Wmax and fast convergence */
+	if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
+		ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
+			/ (2 * BICTCP_BETA_SCALE);
+	else
+		ca->last_max_cwnd = tp->snd_cwnd;
+
+	ca->loss_cwnd = tp->snd_cwnd;
+
+	return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
+}
+
+static u32 bictcp_undo_cwnd(struct sock *sk)
+{
+	struct bictcp *ca = inet_csk_ca(sk);
+
+	return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd);
+}
+
+static u32 bictcp_min_cwnd(struct sock *sk)
+{
+	return tcp_sk(sk)->snd_ssthresh;
+}
+
+static void bictcp_state(struct sock *sk, u8 new_state)
+{
+	if (new_state == TCP_CA_Loss)
+		bictcp_reset(inet_csk_ca(sk));
+}
+
+/* Track delayed acknowledgment ratio using sliding window
+ * ratio = (15*ratio + sample) / 16
+ */
+static void bictcp_acked(struct sock *sk, u32 cnt)
+{
+	const struct inet_connection_sock *icsk = inet_csk(sk);
+
+	if (cnt > 0 && icsk->icsk_ca_state == TCP_CA_Open) {
+		struct bictcp *ca = inet_csk_ca(sk);
+		cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
+		ca->delayed_ack += cnt;
+	}
+}
+
+
+static struct tcp_congestion_ops cubictcp = {
+	.init		= bictcp_init,
+	.ssthresh	= bictcp_recalc_ssthresh,
+	.cong_avoid	= bictcp_cong_avoid,
+	.set_state	= bictcp_state,
+	.undo_cwnd	= bictcp_undo_cwnd,
+	.min_cwnd	= bictcp_min_cwnd,
+	.pkts_acked     = bictcp_acked,
+	.owner		= THIS_MODULE,
+	.name		= "cubic",
+};
+
+static int __init cubictcp_register(void)
+{
+	BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);
+	return tcp_register_congestion_control(&cubictcp);
+}
+
+static void __exit cubictcp_unregister(void)
+{
+	tcp_unregister_congestion_control(&cubictcp);
+}
+
+module_init(cubictcp_register);
+module_exit(cubictcp_unregister);
+
+MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("CUBIC TCP");
+MODULE_VERSION("2.0");